This invention belongs to the field of emulsion chemistry. In particular, it relates to latex polymer compositions containing protonated amines for controlled crosslinking. The latexes are useful in a variety of coating formulations.
In an increasing number of industries, aqueous coating compositions continue to replace traditional organic solvent-based coating compositions. Paints, inks, sealants, and adhesives, for example, previously formulated with organic solvents are now formulated as aqueous compositions. This reduces potentially harmful exposure to volatile organic compounds (VOC's) commonly found in solvent-based compositions. While the move from organic solvent-based to aqueous compositions brings health and safety benefits, the aqueous coating compositions must meet or exceed the performance standards expected from solvent-based compositions. The need to meet or exceed such performance standards places a premium on the characteristics and properties of waterborne polymer compositions used in aqueous coating compositions.
Waterborne polymer having various functional groups have been used to impart and achieve desired properties to a particular coating composition. For example, a coating composition should exhibit good film formation, print and block resistance, as well as adhesion and tensile properties. Polymers having acetoacetyl functional groups represent one example of waterborne polymers which have such properties, may carry different functional groups, and are useful in aqueous coating compositions.
The latex industry has had a long-standing goal to develop effective one-pack crosslinking systems. The ideal system would allow film formation before substantial crosslinking took place. This type chemistry should be unreactive (or very slow to react) in the wet state, but very reactive (at room temperature) in the dry state; referred to hereinafter as latent crosslinking. The result of latent crosslinking would be a good film-forming latex with excellent solvent resistance and hardness.
Several patents have been granted on various one-pack chemistries, including many based on epoxies (glycidyl methacrylate), silanes, isocyanates, and carbonyls (including acetoacetoxy ethyl methacrylate, AAEM). Most of these patents have indicated the presence of crosslinking by demonstrating improved solvent resistance. However, the same results might be expected if the reaction took place in the wet state before film formation, and there is no reason to believe that these systems react through latent crosslinking.
In order to increase the potlife of compositions containing acetoacetate and amine groups it has been proposed to block the amine groups of the polyamine with a ketone or aldehyde to form corresponding ketimine or aldimine compounds prior to mixing with an acetoacetate-functional polymer. Examples of such non-aqueous compositions are disclosed in U.S. Pat. No. 4,772,680. Even though improved stability may be achieved by specific aromatic aldimines, volatile by-products are still formed and the compositions have no application in waterborne coatings and are restricted to coatings using organic solvents as the carrier.
WO 95/09209 describes a crosslinkable coating composition comprising an aqueous film forming dispersion of addition polymer comprising acetoacetate functional groups and an essentially non volatile polyamine having at least two primary amine groups and wherein the mole ratio of acetoacetate to primary amine groups is between 1:4 to 40:1.
EP 555,774 and WO 96/16998 describe the use of carboxylated acetoacetoxyethyl methacrylate latexes mixed with multifunctional amines (such as diethylene triamine) for a shelf-stable, one-component system. In EP 555,774, the system is stabilized by using vinyl acid polymerized with AAEM and the latex is "neutralized" with a polyamine. The patent teaches that the carboxyl groups should be 70 to 96 mol percent relative to the acetoacetoxy groups. WO 96/16998 similarly describes a polymerization process with the vinyl acid and AAEM being polymerized in the first stage.
EP 744,450 describes aqueous coating compositions containing acetoacetate functional polymers with a weight-averaged molecular weight of 100,000 or greater and which contain acetoacetate functional groups and acidic functional groups, and multifunctional amine.
EP 778,317 describes an aqueous self-crosslinkable polymeric dispersion comprising a polymeric component (a relatively hydrophobic polymer having a Hansch number&gt;1.5, at least 5% of a carbonyl functional group capable of reacting with a nitrogen moiety, and at least 1 % of a non-acidic functional group having hydrogen-bondable moieties); and a crosslinking agent comprising a nitrogen-containing compound having at least two nitrogen functional groups capable of reacting with a carbonyl functional moiety. Again it is reported that no gellation has taken place after ten days at 60.degree. C.
U.S. Pat. No. 5,498,659 discloses a single-package aqueous polymeric formulation consisting essentially of an evaporable aqueous carrier, at least one polymeric ingredient having acid-functional pendant moieties able to form stable enamine structures, a non-polymeric polyfunctional amine having at least two amine functional moieties, and an effective amount of base for inhibiting gellation. It is stated in the patent that at least some of the crosslinking of the composition may take place in the liquid phase, possibly within one to four hours of adding the non-polymeric polyfuctional amine. It is postulated that addition of base to the reactor contents competes with the amine-functional moieties vis-a-vis the acetoacetoxy-type functional moieties, thereby reducing the degree of crosslinking and/or enhancing the colloidal stability of the polymer dispersion which forms when the crosslinking reaction takes place.
Geurink, et al., "Analytical Aspects and Film Properties of Two-Pack Acetoacetate Functional Latexes", Progress in Organic Coatings 27 (1996) 73-78, report that crosslinking of acetoacetate functional latexes with polyamine compounds is very fast, and that this crosslinking is hardly hindered by existing enamines. It is further stated that there are very strong indications that crosslinking takes place rapidly in the wet state, in or at the surface of the particles just after mixing of the components. They conclude that as a result of crosslinking in the particles, the film forming process is hampered.
In the patents and articles described above, the usable pot life of the latex formulations is demonstrated by lack of gel formation. It is quite possible, however, that crosslinking is taking place within each particle, without causing the latex to coagulate or gel (e.g. loss of colloidal stability). This type of intraparticle crosslinking (before drying) limits the ability of the latex to form a film upon drying. This in turn reduces the film integrity and performance of the polymer. Therefore, a need still exists for truly latent crosslinking systems--those in which intraparticle crosslinking is inhibited until after film formation. In particular, a need exists for one-pack, latent crosslinking systems which are useful in a wide range of latex applications. These would include decorative and protective coatings, adhesives, non-woven binders, textiles, paper coatings, inks, etc. In each case, the advantage would be a soft, ductile polymer that converts to a harder, more resistant latex film after drying.